A Zone is a region of space within which magic can exist, and is a concept common to all varieties of the D-Verse, unless otherwise specified.
The D-Verse exists as a place normally devoid of magic - which is to say, magic is not normally possible within the unmodified laws of nature and reality. However, within certain special areas of space known as Zones, the mechanisms that cause magical phenomena to manifest in reality can operate properly. Outside Zones, those mechanisms are present but simply do not function. The end result of this is that Zones can be created with relatively minor effort; and judging by the very existence of the mechanisms, the Source always intended for magic to be possible - just not ubiquitous.
The original creation of Zones was an experiment to find an answer to the question "What happens when sentient life is constantly surrounded by the possibility of magic?" by a young Tieria. A large number of worlds were gifted with devices which, when placed in a planetary core, would project a Zone large enough to cover the world up to and including any sort of atmosphere. Because of this, the worlds in question developed considerably differently than those without magic, not only their inhabitants but also their landscapes - providing the God of Knowledge with a rich variety of data.
There are two types of Zones, interim and permanent.
Interim Zones are created to allow magic to operate where a permanent Zone is not in place or is otherwise not available. The creation of this type of Zone is always linked to a particular spell, thus once the spell's effects are complete - or the spell is interrupted - the Zone will collapse.
Permanent Zones are created solely through the use of rune magic. 'Permanent' is something of a misnomer, as the Zones created this way can be turned on and off with the proper sequence of runes; however, unlike interim Zones, these will only collapse when they are turned off or when the Zone generator is destroyed.
Every Zone has a certain level of potency, known as field strength, which is essentially a measure of how difficult it is to counter it. The only way of countering a Zone, apart from interrupting the spell (interim) or deactivating/destroying the generator (permanent) is to set up an opposing field also known as an anti-Zone or negative Zone. The stronger the Zone, the stronger an anti-Zone must be to cancel it out.
In general, interim Zones have fairly low field strength, but it takes considerably less effort to interrupt the attached spell than to set up an anti-Zone that matches the interim Zone. Interim Zones, being temporary, are difficult to locate as they exist only momentarily (for the most part) and are completely different next time they are generated due to changes in the environment.
Permanent Zones are almost universally spherical in nature, as that is the easiest shape to generate. Their field strength is extremely high near the generator and tapers out to near nothing at the edge. Because of this, it takes considerably more power to generate an anti-Zone as one progresses closer to the center of the Zone. Planetary Zones, while (by necessity) incredibly strong, are still relatively weak at their edges; this allows for the formation of anti-Zones if their negative field strength is high (or rather, low) enough to offset the Zone of the planet in question.
Field strength does nothing to the potency of magic used within it; the theoretical limit of 1.0 at the center of a Zone is precisely as effective as the 0.001 or less found at the edge. As long as the field strength is a positive number, the mechanisms that support magic's existence will function. The only difference is the negative field strength requirement for the formation of an anti-Zone.
Zones and rune magic Edit
Rune magic is a special case, as runes themselves bypass the entire concept of Zones and the mechanics therein, functioning instead on the concept of Name. Just as in the Divine Language, a properly Named object or event exists by virtue of its Name alone, and the other laws and mechanics of reality simply bend and flex as necessary to accommodate it.
Though runes may be considered to form an interim Zone to allow their effects for rationalization purposes, no such Zone actually exists, as may be proven by Naming something outside a Zone. Where countering other magic may be done by removing a Zone, only runes may counter runes (by Renaming or Unnaming).